Elsevier

Catalysis Communications

Volume 95, 10 May 2017, Pages 1-5
Catalysis Communications

Short communication
Improving the visible-light photocatalytic activity of interstitial carbon-doped TiO2 with electron-withdrawing bidentate carboxylate ligands

https://doi.org/10.1016/j.catcom.2017.02.025Get rights and content

Highlights

  • Carbon-doped TiO2 was prepared by a novel vapour-assisted solvothermal approach.

  • Bidentate carboxylate ligands can induce band tail states above the valence band edge of TiO2.

  • Bidentate carboxylate ligands can shift the valence band edge towards positive direction.

Abstract

Interstitial carbon-doped anatase TiO2 with bidentate carboxylate ligands (A-C/TiO2) has been prepared through a novel vapour-assisted solvothermal approach and subsequent low temperature calcination process. The bidentate carboxylate ligands could not only strengthen the interaction between the interstitial carbon dopants and TiO2 lattice to narrow the bandgap and enhance the visible-light absorption, but also give rise to the positive shift of the VB edge and cause further bandgap narrowing by inducing the band tail states above the VB edge. These synergistic effects could significantly improve the visible-light photocatalytic activities of interstitial carbon-doped TiO2 on phenol and MO degradation.

Introduction

TiO2-based photocatalysis as a green chemistry technology has drawn considerable attention due to its extensively potential applications in the fields of organic pollutant degradation and solar energy conversion [1], [2], [3], [4]. However, the large bandgap of TiO2 (larger than 3.0 eV) restricts its photocatalytic applications to the ultraviolet range. Therefore, intensive efforts have been made to improve the optical absorption of TiO2 in the visible-light region. Interstitial carbon doping (i.e. carbonate doping) has shown the great potential in introducing such absorption [5], [6], [7], [8], [9]. Previous results have shown that carbonate doping could effectively narrow the bandgap of TiO2 [8], which was responsible for the remarkable visible-light photocatalytic activity. It is well known that the key to bandgap narrowing lies in the introduction of electronic states into the bandgap of TiO2. In general, unlike substitutional carbon doping (i.e. carbon occupying O sites), the interstitial carbon cannot directly introduce p states to narrow the bandgap by mixing with O 2p [10]. Therefore, the introduction of electronic states and the bandgap narrowing were largely dependent on the linkage mode between interstitial carbon and TiO2 lattice. At this point, previous studies always assumed that the structure of carbonate doped TiO2 had three Csingle bondO bonds which were formed by connecting one carbon dopant with three lattice oxygen atoms [8], [10] (this is why interstitial carbon doping was also called carbonate doping). Nevertheless, considering the diversity of oxygen-containing carbon species, there were still the other possible linkage modes between interstitial carbon and lattice oxygen. However, these linkage modes have never been specifically designed and discussed until now.

Due to the strong covalent binding ability to Ti cations, carboxylic groups (COOH) are often employed to act as the anchoring group for dye-sensitized solar cells with a coordination of carboxylate ligands (COO) to surface Ti cations [11]. For the same reason, carboxylic acids are also often employed as both stabilizing solvent and chemical modifier of titanium alkoxides to reduce the reactivities of the precursors in the synthesis process of TiO2 [12], [13]. In this regard, carboxylate ligands might be ideal for the linkage between interstitial carbon and TiO2 lattice, and its strong covalent binding ability might strengthen the interaction of both carbon dopants and Ti cations. Moreover, it is noteworthy that the carboxylate groups deriving from the coordinated carboxylic groups were strong electron-withdrawing substituents. It has been reported that attaching electron-withdrawing substituents onto the polymer main chains represented the most straightforward method for narrowing the bandgap of semiconducting polymers [14]. However, whether the electron-withdrawing carboxylate ligands are still effective for the bandgap narrowing of TiO2 still needs to be further investigated in detail. Therefore, it is highly desirable to explore the construction of the carboxylate linkage between interstitial carbon and TiO2 lattice, the new physical properties and chemical functions arising from the carboxylate linkage and the narrowing bandgap based on the electron-withdrawing carboxylate ligands.

Here it was reported that the visible-light photocatalytic activity of interstitial carbon-doped TiO2 can be further significantly improved with the existence of bidentate carboxylate linkage. Acetic acid (AcOH) was involved as the chelating agent to supply carboxylate ligands. The carbon source was the organic carbon species produced during the synthesis process. The effects of the carboxylate ligands on the morphology and structure of TiO2 were characterized by various techniques. Finally, significantly enhanced visible-light photocatalytic activity for the degradation of phenol and methyl orange (MO) was demonstrated for the obtained A-C/TiO2.

Section snippets

Experimental

All chemicals were used as received without further purification. The interstitial carbon doped TiO2 with bidentate carboxylate linkage (denoted as A-C/TiO2) was prepared through a novel vapour-assisted solvothermal approach and subsequent low temperature calcination process using titanium isopropoxide (TTIP) as the titanium source. The detailed procedure is as follows: 2 mL of TTIP in a 10 mL crucible was placed into a 100 mL Teflon liner, which was then placed into a stainless steel autoclave.

Results and discussions

Fig. 1 shows the XRD patterns of A-C/TiO2 and W-C/TiO2. All diffraction peaks can be exclusively ascribed to TiO2 crystals with the tetragonal anatase phase (JCPDS No. 99-0008). The sharp and intense diffraction peaks of W-C/TiO2 suggest that its crystallinity and particle size were relatively high than those of A-C/TiO2. As Scherrer's equation [15], it was estimated that the particle sizes were 24.0 and 15.9 nm for W-C/TiO2 and A-C/TiO2, respectively. The lower crystallinity and smaller

Conclusions

In summary, through a novel vapour-assisted solvothermal approach and subsequent low temperature calcination process, the carboxylate ligands derived from the chelating AcOH were incorporated into the interstitial carbon-doped TiO2 as the bidentate chelating linkage mode. The bidentate carboxylate ligands can not only strengthen the interaction between the interstitial carbon dopants and TiO2 lattice to narrow the bandgap and enhance the visible-light absorption, but also lead to the positive

Acknowledgements

The authors express their great thanks for the supports from the National Natural Science Foundation of China [Grant No. 20966006], Natural Science Foundation of the Inner Mongolia Autonomous Region [Grant No. 2014MS0218] and the Program for Innovative Research Team in Universities of Inner Mongolia Autonomous Region (NMGIRT-A1603).

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